A variety of manufacturing methods for integrated resistors is known from the prior art. It is convenient to make these resistors in the same process steps as the rest of the integrated circuit. Therefore, the exact way in which the resistors are made depends on which process is used to produce the integrated circuit.
Integrated circuits are generally fabricated with polysilicon resistors that are formed on the semiconductor substrate. Such resistors generally have a resistor body, formed of doped polysilicon and having metallic leads coupled to opposing ends of the resistor body, generally though contacts in field oxide. The contacts are connected to metal interconnect. The resistor body can be formed concurrently with polysilicon transistor gate electrodes, such resistor body generally doped and resting on the field oxide.
Integrated circuits that require passive resistors often have tight tolerances on the resistance value of these resistors. However, these prior art semiconductor resistors are subjected to variations in resistance value. Sources of variation in the resistance value of theses resistors include process fluctuations that result in physical changes to the resistor properties such as physical dimension or resistivity and changes in temperature. The sources of change in temperature can be either external to the resistor itself or internal due to the self-heating effects associated with power dissipation. As the resistor temperature changes, the value of resistance of the resistor also changes.
The general prior art method utilized for minimizing the resistance alteration effects due to the temperature coefficient of resistance (TCR) of semiconducting resistor (a resistor formed of semiconductor material) is to increase the doping concentration in the resistor body to a sufficiently high level such that the TCR of the resistor body is at a minimum. Then the resistors are built with dimensions that make the resistor head resistance a small percentage of the resistor body resistance. As a result, the resistor heads TCR has little effect on the overall resistor temperature characteristics.
A corresponding integrated circuit with an integrated resistance having a minimized temperature coefficient is shown in U.S. Pat. No. 6,333,238. Another method for producing a semiconductor integrated circuit with an integrated resistor is known from U.S. Pat. No. 6,329,262.
Further it is known to use integrated resistors for digital to analogue converters. U.S. Pat. No. 5,604,501 shows a digital-to-analog converter including a resistor string having intermediate taps at resistor junctions as well as resistor-potential junctions. Switching transistors are coupled between a respective intermediate tap and an output node. Decode circuits are capable of switching at least two transistors to be in the on state at the same time to electrically couple more than one intermediate tap to the output node to produce at least one analog output. One row select line can be energized simultaneously with at least two column select lines. Alternatively, at least two row select lines can be energized simultaneously with one column select line.
U.S. Pat. No. 6,278,393 shows an integrated circuit including a digital-to-analog converter in which a resistor string is adapted to be coupled to a reference source. The resistor string includes a plurality of serially coupled impedances defining intermediate taps at the junctions thereof. A first plurality of switches are coupled between a first output node and respective ones of the intermediate taps.
A first selection circuit receives a first digitally codes signal and is coupled to each switch in the first plurality of switches. The first selection circuit selectively switches the first plurality of switches to predetermined states depending upon a first digitally coded signal provided thereto, to generate a first analog output. A second plurality of switches are coupled between a second output node and respective ones of the intermediate taps. A second selection circuit coupled to each switch in the second plurality of switches selectively switches the second plurality of switches to predetermined states depending upon a second digitally codes signal provided thereto, to generate a second analog output.
Generally the resistance tolerance of an integrated resistor is within about ±20%. Integrated resistors having a high square resistance have even higher tolerances. The lack of a method to produce integrated resistors having a precise resistance value has limited the field of use of integrated resistors in the prior art.